Time base combining radioactive source and solid-state detector

ABSTRACT

A radioactive timekeeping standard constituted by a radioactive source of alpha particles combined with a solid-state radiation detector, the source being in the form of a backing having a planar array of discrete islands of a radioactive isotope, the alpha particles emitted therefrom passing through an apertured mask and impinging on the sensitive surface of a solid-state radiation detector, the geometry of the mask apertures, which are in a matching array, being such as to restrict emanations impinging on the detector surface to substantially normal angles of incidence and in addition preventing particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island.

I United States Patent nu 3,582,656

I [72] Inventor Dale R. Koehler 3,223,842 12/1965 Hyde I 250/106S RiverVale. NJ. 3,370,414 2/1968 Lazrus et al. 250/83.3X [2H p 714354 PrimaryExaminer-Walter Stolwein {l k d ir Assistant Examiner-Morton J. Frome ld 5* e Att0rney-Michael Ebert [73] Assignee Bulova Watch Company, Inc.

New York, N.Y.

radioactive standardconstit uted AND SOLIDSTATE DETECTOR by a radioactve source of alpha particles combined with a H Claims, 6 Drawing Figssolid-state radiation detector, the source being in the form of abacking having a planar array of discrete islands of a radioac- [52]U.S.Cl 250/83.3, five isotope the alpha particles emitted therefrompassing 58/23, 250/83 250/105i 250/106 through an apertured mask andimpinging on the sensitive sur- [51] Int. Cl 0. G01! 1/24 f of a so|idstate radiation detector, the geometry f the [50] Flcld of Search250/83.3, mask apertures, whizh are in a matching array being Such as106 (s); 58/23 to restrict emanations impinging on the detector surfaceto [56] References Cited substantially normal angles of incidence and inaddition preventing particles emanating from any one island from UNITEDSTATES PATENTS impinging on a neighboring portion of the detectorsurface as- 2,683,8l3 7/1954 Friedman 250/ 106 sociated with anotherisland.

, MASK /3 138 130 H 10 Ww/cwwvf I J I [Ara/e) L \\\'\t\' x'\\"\'\t\t\\r'\\\\ \t T\\\\\ TIME BASE COMBININGIRADIOACTIVESOURCE ANDSOLID-STATE DETECTOR Related Applications: (A) Ser. No. 651,864, filedJuly 7, 1967, now abandoned of Koehler and Grissom, entitled Timepiecewith Radioactive Timekeeping'Standard" and (B) Ser. No. 700.l02, filedJan. 24, I968, of Koehler, entitled Multicellular Solid-State RadiationDetector Assembly.

This invention relates generally to radioactive time bases, and inparticular to a timekeeping standard constituted by a radioactive sourceof alpha particles combined with a solidstate radiation detector.

In the above-identified application (A), there is'dis'closed a timepiecearrangement in which a radioactive source having a relatively longhalf-life emits charged particles which are sensed by a solid-stateradiation detector. The detector yields a relatively large number ofelectrical pulses per second, the pulses being scaled down 'byelectronic pulse frequency dividers to produce a low number of controlpulses," such as one per second. The periodic control pulses areiappliedtoan electronic or electromechanical time register 'to'actuate orcontrol the register to indicate time.The combinationof the radioactivesource and detector is=designated a timekeeping standard,as-distinguished from the associated pulse scaling and indicatingstages.

Although nuclear disintegrationsare distributed randomly in time, timingaccuracy can be obtained through the'accumulation of a'sufficient numberof counts.Since the disintegrations obey the Poisson distribution inaccordance with probability theory, one'can calculatethe' statisticalaccuracythat can be expected from'a total-nuinberof counts, assumingthat the counting system contributes negligible'error.

As pointed out in copending application (A), the'p'referred form ofradioactive source for the timekeeping-standard is an isotope whichemits alpha particles and has a prolonged halflife. Whilegamma' rays areradiated with-discrete energies,

and in that respect are nearly mon'oenergetic andca'n-beused for timingpurposes, they are a-highly penetrating formof radiation; hence it wouldnot be feasible, within the confines of a watch or small'timepiece, toprovide the necessary protective shielding. Also, it would not bepossible withigam'ma rays to control the area of the detector to beexposed to the radiation source.

Beta particles, on the other'hand, are not emitted with discreteenergies, but have a continuous distribution 'of energies. Thisradiation is a high-speed "electron that is emitted at thetransformation of-a neutron to aproton within the nucleus of an atom.While it is possible to effect shielding of beta parti cles with a fewmillimeters of aluminum, timing control is very difficult since theparticles are not monoenergetic.

The reason for this is thatthe output pulse heights ofa solidstateradiation detector are proportional to the ionization produced byincident radiation-Each nuclear particle of the same type will loseapproximately the same proportion of energy through the ionizationprocess,'thereby establishing a direct relationship betweenthe pulseheight'of the detector signal and the energy of the radiation. Unlessthe radiation is nearly monoenergetic, electronic instabilities inthesystem can cause variations in the detection of low energy pulses and itbecomes difficult todiscriminatebetween detector output pulses andelectrical noise inherent in solid-statedetectors and associatedamplifiers. This gives rise to undesirable'variations in timekeeping.

Alpha particles consist of two protons and'two neutrons,

and possess a charge twice that of an electron but opposite in sign, asis also'the case for a nucleus of a helium atom. The quantity of energyreleased is discrete, itsmagnitude being characteristic of theparticular alpha .particle emitting radioisotope'Naturally radiatedalpha particles have energies 'ran in from a roximatel 4 to m.e.v.' Thefact that alpha radiation is highly ionizing accounts 'for itsrelatively short range whentraversingmatter. This range'is only'afew'centimeters in standard-air, and several sheets of'ordi'narypaperwill absorb even the most energetic of alpha particles. Yet

from the characteristic properties of gammagbeta and alpha particles, itis evident that only alpha particles are suitable for radioactivetimekeeping standards, for not only are they nearly monoenergetic, butthey can be handled in a practical sense within the confines ofa smalltimepiece.

When alpha particles are radiated from a relatively thick source, alphaparticle energies are absorbed within the thickness of'the radioisotopedeposit itself. Thus a continuous distribution of energies will resultfrom alpha particles being radiated from various depthsin thethick'layer. The spread of this'distribution can be minimized byobtaining the required activity from the thinnest source possible.

Another cause'of spread or departure from monoenergicity, is the airgapbetween the radioactive isotope and the detector. While thiscan'theoretically be overcome by placing the detector and source in avacuum, this solution is not practical. A

contact with the detector. However, existing semiconductive solid-statedetectors are physically constructed with a thin entrance window throughwhich the particulate radiation must pass before entering the sensitivevolume or depletion zone of the detector. Though one can make thiswindow very thin, the intimate contact geometry results in varyingdegrees of energy degradation. At small angles of incidence, thisdegradation reachesa level equal to the total energy of the incidentparticles.

One solution to the entrance angle effect is to displace the source fromthe detector by a distance which is such as to admit only radiationwhose angle ofincidence is about normal to the detector surface.However, this remedy is not practical in a small timepiece because ofphysical size limitations.

In view of the foregoing, it is the primary object of my invention toprovide a time base constituted by a radioactive confine emanation fromthe source as to maintain it nearly monoenergetic.

Also an object of the invention is to provide an efficient and reliableassembly of radioactive source and solid-state radiation detector.

Briefly stated, these objects are accomplishedin a radioactive time baseassembly comprising a backing-having an array of discrete islandsthereon of a radioactive isotope emitting alpha particles and having arelatively protracted'half-life, a

mask being interposed between the islands and the surface of asolid-state radiation detector, the mask having a matching array ofapertures therein whose geometry is such as to conline the particulateenergyimpinging on the detector surface to substantially normal anglesof incidence and to prevent particles emanating from any one island fromimpinging on a neighboringportion of the detector surface associatedwith another island.

Fora better'understanding of the invention, as well as other objectsand-furtherfeatures thereof, reference is had to the following detaileddescription toberead inconjunction with the accompanying drawing,wherein:

FIG. 1 schematically illustrates an assembly composed of a single layerof radioactive material and a detector, anapertured mask beinginterposed therebetween, this illustration being'for purposes ofbackground analysis;

FIG. 2 schematically illustratesthe behavior of the device shown inFIGpl;

FIG. 3 schematically shows a radioactive timekeeping standard inaccordance with the invention;

FIG. 4 illustrates the behavior of the standard shown in FIG.

7 FIG. 5 is an exploded perspective view of an assembly of the typeshowninFIG. 3; and

FIG. 6 is a modified form of standard in accordance with the Iinvention.

RADIOISOTOPES The requirements for an alpha-emitting radioactive isotopewill now be considered. Although many radioisotopes with natural alpharadiation'are commercially available, most of them are not suitablebecause their half-lives are not sufficiently protracted to satisfy thehalf-life requirements for a timekeeping standard as set forth incopending application (A). The following radioisotopes are consideredsuitable for timekeeping purposes, in addition to those alreadyidentified in said copending application:

Half-life Radioisotope: (year) Plutoniuni239 2. 436 Uranium-238 4. 51X10 Uranium 235 7. 1 10 Neptunium 237 2. 2X 10 As shown in FIG. 1, alayer 10 of the selected radioisotope is formed on a backing 11, whichmay be of platinum or alu minum, or any other material providingadequate support and preferably having shielding properties. To minimizethe spread of energy distribution, the layer is made as thin and asuniform as possible. To this end, a deposition technique may beemployed, the radioactive material being laid down in a very dilutesolution on the backing and then allowed to dry, the resultant filmadhering to the backing.

Detector 12, which is used to intercept alpha particles emanating fromlayer 10, may be of the surface barrier or diffused-junction typecommercially available. While the present invention resides in the useof an apertured mask in combination with a radioactive source in theform of an array of separate islands of radioisotope material, the mask13 in FIG. I is shown in combination with a single, continuousradioactive layer 10, and is provided with an array of apertures 13A,13B, 13C, etc., defining parallel passages of uniform cross section forthe emanations. This combination is not in accordance with theinvention, but is shown only for purposes of background analysis.

FIG. 2 indicates the trajectories of particles emanating at variousangles from different points on source 10, and traveling towardthesurface of detector 12. Path P is normal. to the surface of detector 12.This is the shortest and most direct path and provides maximum energy.The angle of incidence of path P is such that it passes through theupper edge of the mask, some energy being absorbed therein, whereby theremaining energy of the particles arriving at the detector is reduced.Path P which cuts through the lower edge of the mask, is even furtherreduced in energy. Similarly, paths P P and P are intercepted by varyingthickness of the solid body of the mask, and are more or less reduced inenergy by absorption.

Thus the particles in path P will produce a relatively large outputpulse in the detector, whereas those in the other paths will producepulses having lesser and varying degrees of amplitude. Hence while thealpha-emitting source is nearly monoenergetic, the detector responds asif the source had a spread of energy distribution, which is undesirablefor timekeeping purposes.

OPERATING PRINCIPLES OF THE TIMEKEEPING STANDARD Each island 14A, 143,etc. is centered with respect to the upper zone of the correspondingaperture defined by the upper section I, which upper zone has arelatively large and uniform cross section, the diameter of the islandbeing equivalent to or less than that of the upper zone. The lower zoneof the aperture defined by the lower section II, has at its top side asmaller diameter preferably equal to or greater than the diameter of theassociated island, the underside of the aperture being chamfered toprovide a flared mouth of increasing cross section.

The preferred geometry of the mask structure eliminates those eventswhich would cause an energy loss in the aperture edge adjacent theradioactive island. Thus it will be seen that the trajectories indicatedby emission paths Pa, Pb and'Pc normal to the detector surface, areunobstructed by the'mask. Paths Pd and Fe, which represent very lowangles of incidence are intercepted by the upper section I of the maskand completely absorbed thereby.

Because of the flared lower edges of the apertures, paths Pf, Pg and Ph,which are not normal but which have relatively high angles of incidence,go directly to the detector surface without striking an aperture edgeand hence without being degraded. Virtual elimination of thedetector-side aperture edge by flaring causes a minimization of thesolid angle subtended by absorptive mask material at the source, thusminimizing the number of particles that can actually be energy degradedin the mask material.

Thus the apertured mask in accordance with the invention preventsparticles from any one island from impinging on a neighboring portion ofthe detector surface at a low angle of incidence, and provides anentrance aperture subtending an optimum angle at the detector. Thegeometry of the aperture in the mask is such as to minimize edge effectsas well as to reduce airgap losses.

STRUCTURE OF TIMEKEEPING STANDARD Referring now to FIG. 5, there isshown a practical embodiment of a timekeeping standard in accordancewith the invention. Backing 11 for the radioactive source is in the formof a thin disc of suitable shielding material on which is deposited auniform array of thin circular islands 14A, 143, etc., of radioactivematerial possessing alpha-particle-emitting properties. The islands areconstituted by the deposits of radioactive material substantiallyequispaced from each other.

Mask 13 includes a circular upper plate I having relatively largeapertures in a configuration matching the array of the islands, thediamet er of the plate being equal to that of the backing 11. Mask 13 isprovided also with a lower plate I] having a corresponding array ofsmaller apertures whose underside (not shown) is flared, as indicated inconnection with FIG. 3. Finally, below plate II is a disc-shapedsolid-state radiation detector 12.

When the four discs are brought together, the resultant waferconstitutes a highly compact and efficient timekeeping standard whichmay readily be incorporated in a small timepiece or watch. In astructure of this type, the geometry of the mask is such as to restrictemanations impinging on the detector surface to nearly normal angles ofincidence and furthermore preventing particles emanating from any oneisland from impinging on a neighboring portion of the detector surfaceassociated with another island.

Preferably, the diameter of each island or the diameter of the circlecircumscribing the island, should the island not be circular, is nogreater than twice the distance between the sur face of the island andthe plane of the detector, the diameter of each mask aperture being notless than the diameter of the island or the circle.

MODIFIED FORM OF TIMEKEEPING STANDARD In the conventional solid-stateradiation detector, an electric field is set up across a lowconductivityregion, which region is the charge depletion layer at the diode junctionoperating at reverse bias. When a charged particle passes through thesemiconductive medium, electron hole pairs are produced therein. Thesecharges are caused to separate by the electric field and the resultantelectrical signal can be transmitted to a measuring system to afforduseful information respecting the particles detected.

The principal drawback in existing solid-state detectors is that itssensitivity, especially to low-energy particles, tends to be very low,for there is an appreciable probability of absorption of such particlesbefore they reach the depletion layer, and even ifa pair of charges isproduced in the depletion layer, the quantum efficiency is limited toone pair per particle, with no chance of multiplication such as iseffectively obtained in Geiger-Muller tubes and proportional counters.

The low sensitivity dictates the use of high-gain amplifiers. Thus inthe case of detectors 12 shown in the previous figures high-gainamplication is necessary. However, the output signal from a conventionalsolid-state radiation detector lies in the millivolt range and is notmuch more pronounced in amplitude than the noise level in the associatedelectronic amplifying circuits for elevating the signal to a levelsuitable for measurement and analysis. This noise may give rise tospurious signals which cannot readily be distinguished from theradiation signals, thus adversely affecting the sensitivity and energyresolution of the detection system.

In my copending application (B), there is disclosed a multicellular,solid-state radiation detector assembly adapted to produce exceptionallylarge signals in response to incident radiation, the detector beingconstituted by an array of individual surface-barrier ordiffusedjunction, radiation-sensitive, semiconductive cells, each ofwhich has a small area and a low internal capacitance.

The cells in the array are unidirectionally connected in parallelrelation with respect to current flow, but are otherwise electricallyisolated from each other, whereby the overall capacitance of the arrayis low while the detection efficiency thereof is substantially equal toa unitary radiation detector whose surface area is equivalent to theaggregate area of the cells, the signal output from the multicellulardetector being far greater than that yielded by the unitary detector.

In the arrangement shown in FIG. 6, the multicellular solidstateradiation detector is combined with an array of radioactive islands 14A,143, etc., and an apertured mask 13 of the type shown in FIG, 3. Themulticellular detector is constituted by an array of tiny radiationdetector cells 16A, 16B, 16C, 16D, etc., whose diameters aresubstantially the same as that of the radioactive islands and which arepositioned in registration therewith.

Cells 16A, 168, etc., are unidirectionally connected in parallelrelation with respect to current flow by diodes 17A, 17B, 17C, etc., butare otherwise electrically isolated from each other, whereby the overallcapacitance of the array of cells is low, whereby the detectionefficiency thereof is substantially equal to a unitary radiationdetector, such as detector 12, whose surface area is equivalent to theaggregate area of the cells. However, the signal output from themulticellular detector is far greater than that yielded by the unitarydetector. In practice, the parallel-connected detector cells areconnected to an output circuit which imposes a reverse bias thereon.

While there have been shown and described preferred embodiments of myinvention, it will be appreciated that many changes and modificationsmay be made therein without, however, departing from the essentialspirit of the invention as defined in the annexed claims.

What I claim is:

l. A radioactive timekeeping standard adapted to produce substantiallymonoenergetic timing pulses and comprising:

a. a planar array of discrete islands of a radioactive isotope emittingalpha particles, said isotope having a prolonged half-life whoseduration is suitable for timekeeping purposes,

b. a solid-state radiation detector having a sensitive surface inparallel relationship to said planar array, the area of said surfacebeing substantially coextensive with the area of said array, and

. an alpha particle-absorbing mask interposed between said array andsaid surface and having a matching array of alpha particle-admittingapertures whose geometry is such as to restrict the emanations impingingon the detector surface through the spaces in the apertures to nearlynormal angles of incidence and preventing particles emanating from anyone island from impinging on a neighboring portion of the detectorsurface associated with another island to obviate a spread in energydistribution.

2. A standard as set forth in claim 1, wherein said array of islands isdeposited on a backing having protective shielding properties.

3. A standard as set forth in claim 1, wherein said islands are formedof a thin film of radioactive material to prevent a spread of energydistribution.

4. A standard as set forth in claim 1, wherein each aperture in saidmask is composed of a first zone adjacent its associated island having arelatively large cross section and a second zone adjacent the detectorsurface having a constricted cross section.

5. A standard as set forth in claim 4, wherein said second zone of eachaperture is flared outwardly in the direction of the detector surface.

6. A standard as set forth in claim 1, wherein said radioisotope isselected from a class consisting of uranium 238, uranium 235, neptunium237 and plutonium 239.

7. A standard as set forth in claim 2, wherein said backing is a metaldisc and said mask is composed of at least one circular plate of thediameter, said detector also having the same configuration.

8. A standard as set forth in claim 7, wherein said mask is composed oftwo circular plates, one having apertures defining the first zone andthe second having apertures defining the second zone.

9. A standard as set forth in claim 1, wherein said detector is formedby an array of individual cells, each disposed to intercept radiationfrom a respective island, the cells being connected unidirectionally inparallel.

10. A standard as set forth in claim 9, wherein said cells are connectedin parallel through diodes and are reverse biased.

11. A radioactive timekeeping standard comprising:

a. a planar array of discrete islands of a radioactive isotope emittingalpha particles, said isotope having a prolonged half-life.

b. a solid-state radiation detector having a sensitive surface inparallel relationship to said planar array, and

c. a mask interposed between said array and said surface, having amatching array of circular apertures whose geometry is such as torestrict the emanations impinging on the detector surface to nearlynormal angles of incidence and furthermore preventing particlesemanating from any one island from impinging on a neighboring portion ofthe detector surface associated with another island, wherein thediameter of the circle circumscribing such islands is not greater thantwice the distance between the surface of the islands and the plane ofthe detector and furthermore that the diameter of each of said aperturesis not less than the diameter of said circle.

2. A standard as set forth in claim 1, wherein said array of islands isdeposited on a backing having protective shielding properties.
 3. Astandard as set forth in claim 1, wherein said islands are formed of athin film of radioactive material to prevent a spread of energydistribution.
 4. A standard as set forth in claim 1, wherein eachaperture in said mask is composed of a first zone adjacent itsassociated island having a relatively large cross section and a secondzone adjacent the detector surface having a constricted cross section.5. A standard as set forth in claim 4, wherein said second zone of eachaperture is flared outwardly in the direction of the detector surface.6. A standard as set forth in claim 1, wherein said radioisotope isselected from a class consisting of uranium 238, uranium 235, neptunium237 and plutonium
 239. 7. A standard as set forth in claim 2, whereinsaid backing is a metal disc and said mask is composed of at least onecircular plate of the diameter, said detector also having the sameconfiguration.
 8. A standard as set forth in claim 7, wherein said maskis composed of two circular plates, one having apertures defining thefirst zone and the second having apertures defining the second zone. 9.A standard as set forth in claim 1, wherein said detector is formed byan array of individual cells, each disposed to intercept radiation froma respective island, the cells being connected unidirectionally inparallel.
 10. A standard as set forth in claim 9, wherein said cells areconnected in parallel through diodes and are reverse biased.
 11. Aradioactive timekeeping standard comprising: a. a planar array ofdiscrete islands of a radioactive isotope emitting alpHa particles, saidisotope having a prolonged half-life. b. a solid-state radiationdetector having a sensitive surface in parallel relationship to saidplanar array, and c. a mask interposed between said array and saidsurface, having a matching array of circular apertures whose geometry issuch as to restrict the emanations impinging on the detector surface tonearly normal angles of incidence and furthermore preventing particlesemanating from any one island from impinging on a neighboring portion ofthe detector surface associated with another island, wherein thediameter of the circle circumscribing such islands is not greater thantwice the distance between the surface of the islands and the plane ofthe detector and furthermore that the diameter of each of said aperturesis not less than the diameter of said circle.